When developing diesel engines, two important objectives are to reduce emissions and to reduce fuel consumption. Attempts are made to achieve this with on the one hand greater precision when establishing the injection pattern and on the other hand a more homogenous combustion process in the engine. In addition to system hardware modifications, which for example allow the smallest possible quantity of fuel to be injected to be reduced, the pressure in the fuel injection line is also increasingly raised.
The most recent generations of injectors allow multiple injections, resulting in the injector needles undergoing a number of opening and closing operations at very short time intervals. These needle movements produce pressure oscillations in the injection line, the amplitude of which can amount to a deviation of around 10% from the operating point. The precise pattern of pressure fluctuations over time is a function of the selected injection pattern, the injector characteristics, the hydraulic properties of the injection line and the high-pressure pump as well as fuel properties such as temperature and viscosity. The large number of influencing parameters means that pressure fluctuations cannot be calculated analytically at present.
The pressure fluctuations at the injector end of the injection line influence the quantity and pattern over time of the fuel injected. Pressure measurement at this point requires the integration of an additional sensor that can detect dynamic pressure fluctuations with sufficient accuracy. Such a sensor is out of the question for reasons of cost. As such a sensor operates invasively, it also changes the hydraulic properties of the injection line.
Sensors based on the magneto-elastic effect are known. This means that a ferromagnetic material changes its magnetic properties when it is subject to a change in length. This means that the hysteresis curve that describes the non-linear relationship between the magnetic field H and the magnetic flux density B by means of the magnetic permeability μr is extended or compressed (see FIG. 3).
The pressurized fuel produces radial and axial compressive and tensile stresses in the injection line made of ferromagnetic material. A pressure change therefore results in a change in the mechanical stresses in the injection line material. The magneto-elastic effect causes the magnetic permeability of the material μr to change, resulting in a magnetic flux density B that changes approximately in proportion to the pressure fluctuation, when the magnetic field H is constant. A magnetic flux density B that changes over time produces an induced voltage Uind in a coil according to the law of induction. This voltage can be measured and used to reconstruct the pressure fluctuations in the injection line.
The publication DE 101 00 957 A1 discloses a sensor having three wire-wound coils, wound round an injection line. Direct current flows through the two outer coils, thereby producing a magnetic field H that is constant over time. The center coil is used to measure the induced voltage. This is amplified by a signal amplifier arranged in proximity to the sensor. To screen it from external magnetic fields that may have the effect of interfering with the induced voltage, the sensor may be enclosed by a screening housing made of magnetically soft material that is attached to the injection line.
The publication DE 197 57 293 A1 describes a device for determining the start of injection by means of a magneto-elastic sensor, which is used to determine the relative pressure for example.
It is a disadvantage of known sensors that they cannot determine the absolute pressure within an injection line.